Image generating apparatus, image generating method, and program

ABSTRACT

An image generating apparatus of the present invention has a determination unit that sets a target area in a part of an area inside a subject, executes processing to adjust a phase of each detection signal based on a distance from each detection element to a target area and a tentative velocity, and calculate dispersion of intensities of a plurality of detection signals of which phases are adjusted, for a plurality of tentative velocities, and determines a velocity for which dispersion of intensities is minimum, out of the plurality of tentative velocities, as a propagation velocity.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 14/171,337,filed Feb. 3, 2014, which is a divisional of application Ser. No.13/498,192, filed Mar. 26, 2012, which is a national-stage applicationof PCT/JP2010/007150, filed Dec. 8, 2010. This application claimsbenefit of all those applications under 35 U.S.C. § 120, and claimsbenefit under 35 U.S.C. § 119 of Japanese Patent Application No.2009-281452, filed on Dec. 11, 2009. The entire contents of each of thementioned prior applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image generating apparatus, imagegenerating method and program, for generating an image representinginformation inside a subject.

BACKGROUND ART

Research on imaging apparatuses which irradiate light from a lightsource (e.g. laser) onto a subject, such as living body, and visualizethe information inside the subject, has been energetically progressingin medical fields. An example of such a visualization technology usinglight is photoacoustic tomography (PAT). A photoacoustic tomographyapparatus detects an acoustic wave (typically an ultrasonic wave)generated from a tissue of the living body, which absorbed energy of thelight propagating in and diffusing from a subject, at a plurality oflocations surrounding the subject. Then the obtained signals aremathematically analyzed, and the information related to the opticalproperty inside the subject, particularly the absorption coefficientdistribution, is visualized. Recently pre-clinical research on imagingthe blood vessels of small animals using the photoacoustic tomographyapparatus, and clinical research on applying the principle of thephotoacoustic tomography apparatus to diagnose breast cancer or the likeis energetically progressing.

In the case of photoacoustic tomography apparatus and ultrasonicdiagnostic apparatus (apparatus for detecting acoustic waves reflectedin living body and generating an image) which have been conventionallyused in medical fields, images are usually generated using an averageacoustic velocity of the subject (sound speed of acoustic wave insidethe subject, propagation velocity of acoustic wave inside the subject orpropagation speed of acoustic wave inside the subject). Generally soundspeed is determined based on an experiential value or document-basedvalues. However propagation speeds have individual differences, andsound speed also changes depending on the method of holding a subject,for example. Therefore if the sound speed used for generating an imageand the actual sound speed are different, the resolution of an imagedrops considerably.

Patent Literature (PTL) 1, for example, discloses a way to solve thisproblem. According to the technology disclosed in Patent Literature(PTL) 1, sound speed is determined so that brightness or the contrast ofeach pixel (or voxel) is maximized. Thereby a drop in image quality, dueto a mismatch of the sound speed used for generating the image and theactual sound speed, is suppressed.

However in the case of the technology in Patent Literature (PTL) 1, thebrightness or contrast of the background noise also increases since thebrightness or contrast of each pixel is maximized. Furthermore if noiseis included in the detection signals, the sound speed is determined sothat the total value of the noise component and normal signal componentis maximized, therefore an accurate sound speed cannot be obtained, andthe image blurs.

(PTL 1) Japanese Patent Application Laid-Open No. 2000-166925

SUMMARY OF INVENTION

The present invention provides an image generating apparatus and animage generating method which can generate an image representinginformation inside a subject, with suppressing a drop in image qualitydue to noise and a mismatch of the sound speed used for generating theimage and the actual propagation velocity (sound speed).

The present invention in its first aspect provides an image generatingapparatus comprising:

a probe having a plurality of detection elements which detect anacoustic wave propagating from inside a subject and output detectionsignals;

a determination unit that determines a propagation velocity of theacoustic wave inside the subject;

an image generating unit that generates an image representinginformation inside the subject using the propagation velocity determinedby the determination unit and a plurality of detection signals obtainedfrom the plurality of detection elements, wherein

the determination unit sets a target area in a part of an area insidethe subject,

executes processing to adjust a phase of each of the detection signalsbased on a distance from each of the detection elements to the targetarea and a tentative velocity, and calculate dispersion of intensitiesof the plurality of detection signals of which phases are adjusted, fora plurality of tentative velocities, and

determines a velocity for which dispersion of intensities is minimized,out of the plurality of tentative velocities, as the propagationvelocity.

The present invention in its second aspect provides an image generatingmethod comprising:

a step of detecting an acoustic wave propagating from inside a subjectusing a plurality of detection elements and generating detectionsignals;

a determination step of determining a propagation velocity of theacoustic wave inside the subject; and

an image generating step of generating an image representing informationinside the subject using the propagation velocity determined in thedetermination step and a plurality of detection signals obtained fromthe plurality of detection elements, wherein

in the determination step,

a target area is set in a part of an area inside the subject,

processing to adjust a phase of each of the detection signals based on adistance from each of the detection elements to the target area and atentative velocity, and calculate dispersion of intensities of theplurality of detection signals of which phases are adjusted, is executedfor a plurality of tentative velocities, and

a velocity for which dispersion of intensities is minimized, out of theplurality of tentative velocities, is determined as the propagationvelocity.

The present invention in its third aspect provides a non-transitorycomputer readable medium recording a computer program for causing acomputer to perform a method comprising:

a determination step of determining a propagation velocity of anacoustic wave inside a subject; and

an image generating step of generating an image representing informationinside the subject using a plurality of detection signals, which aregenerated by detecting an acoustic wave propagating from inside thesubject using a plurality of detection elements, and the propagationvelocity determined in the determination step, wherein

in the determination step,

a target area is set in a part of an area inside the subject,

processing to adjust a phase of each of the detection signals based on adistance from each of the detection elements to the target area and atentative velocity, and calculate dispersion of intensities of theplurality of detection signals of which phases are adjusted, is executedfor a plurality of tentative velocities, and

a velocity for which dispersion of intensities is minimized, out of theplurality of tentative velocities, is determined as the propagationvelocity.

According to the present invention, an image representing informationinside a subject can be generated with suppressing a drop in imagequality due to noise and a mismatch of the velocity (sound speed) usedfor generating the image and the actual propagation velocity (soundspeed).

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram depicting a principle of the present invention;

FIG. 2 is a diagram depicting an example of a configuration of an imagegenerating apparatus according to the present embodiment;

FIG. 3 is a flow chart depicting a method for generating a generatedimage;

FIG. 4A is a diagram depicting an example of the positional relationshipof a detection element and a processing target position;

FIG. 4B is a graph depicting an example of the directivity of thedetection element;

FIG. 5A is a diagram depicting an example of simulation conditionsaccording to Example 1;

FIG. 5B is a graph depicting an example of a detection signal of thesimulation according to Example 1;

FIG. 6 shows an example of the images generated in simulation accordingto Example 1;

FIG. 7A shows an example of the image generated by an image generatingapparatus according to Example 2; and

FIG. 7B shows an example of the image generated by the image generatingapparatus according to Example 2.

DESCRIPTION OF EMBODIMENTS

<Principle>

The principle of the present invention will now be described.

FIG. 1 is a diagram depicting the principle of the present invention. InFIG. 1, the reference number 1 indicates a sound source (inside asubject), the reference number 2 indicates a voxel (or a pixel), and thereference number 3 indicates a detection element. Each detection elementdetects an acoustic wave propagating from the inside of the subject by atime sharing measurement, and outputs a detection signal. A number ofthe detection elements 3 is N (N is a 2 or higher integer).

Generally, an image generating apparatus using acoustic waves(ultrasonic waves) generates an image representing information inside asubject using a plurality of detection signals S (i, t) obtained from aplurality of detection elements i. This kind of image is called a“generated image”, and is also called “volume data” if the generatedimage is a three-dimensional image (an aggregate of voxels). A generatedimage is also called “pixel data” if it is a two-dimensional image. Eachpixel (or voxel) of a generated image is normally calculated using adetection signal of which phase is adjusted based on a distance fromeach detection element to a position corresponding to the pixel andpropagation velocity (sound speed) (or propagation velocity (soundspeed) of the acoustic wave inside the subject). In the case of aFourier domain method, however, an image is generated by operation in afrequency space. In FIG. 1, i denotes a number (integer in a 0 to N−1range) of the detection element, and t denotes time.

Hereafter the time domain method, which is a general image generationmethod, will be described in concrete terms. First for each detectionelement, a distance from the i-th detection element to a positioncorresponding to a pixel in a generated image is divided by a soundspeed. Using this result, time Ti for an acoustic wave reaches thedetection element i (delay time) when the acoustic wave generated inthis pixel position is calculated (generation time is assumed to bet=0). The intensity S (i, Ti) of the detection signal at time Ti iscalculated for each detection element, and the results are added,whereby the pixels of the generated image are generated (the generatedimage is generated by generating pixels for a plurality of positions inthe same manner). According to the technology disclosed in PatentLiterature (PTL) 1, the sound speed, that is the time Ti, is determinedin this generated image generation method so that the data of each voxel(or pixel) is maximized. The intensity S (i, Ti) indicates the intensityof the detection signal S (i, t) at time Ti (intensity of the detectionsignal of which phase is adjusted).

Here if the sound speed estimated as the sound speed and the actualsound speed are significantly different, the intensity S (i, Ti)disperses greatly. To prevent this, according to the present invention,a target area 5 (area to be targeted) is set in a part of an area insidethe subject. Then processing of adjusting the phase of each detectionsignal S (i, t), based on the distance from each detection element i tothe target area 5 and a tentative sound speed, and calculating thedispersion of the intensities S (i, Ti) of a plurality of detectionsignals of which phases are adjusted, is executed for a plurality oftentative sound speeds. Then a sound speed, out of the plurality oftentative sound speeds, of which the above mentioned dispersion of theintensity S (i, Ti) is minimized, is determined as the sound speed, andthe generated image is generated based on this sound speed.

For example, a coherent factor (CF) given by the following Expression(101) can be used as an index of the dispersion of the delay signalvalue.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\mspace{616mu}} & \; \\{{CF} = \frac{{{\sum\limits_{i = 0}^{N - 1}{S(i)}}}^{2}}{N{\sum\limits_{i = 0}^{N - 1}{{S(i)}}^{2}}}} & (101)\end{matrix}$

Here S (i) is an intensity of a detection signal which is output fromthe detection element i and of which phase is adjusted, that is anintensity S (i, Ti) of the detection signal at time Ti which iscalculated based on a sound speed. If S (i) becomes a same valueregardless the detection element, the index becomes CF=1, and as thedispersion of the value S (i) increases, the index CF approaches closerto 0. In other words, as the dispersion of the intensity S (i, Ti) isgreater, the value of the index CF becomes closer to 0, and as thedispersion is smaller, the value becomes closer to 1. In the presentinvention, a sound speed at which the dispersion of the intensity S (i,Ti) is minimized, that is a sound speed at which the index CF is themaximized is determined to be the sound speed. The index is not limitedto CF. The dispersion may be evaluated with a scale which is used instatistical fields, such as a dispersion value and a standard deviationvalue.

Thus according to the present invention, the sound speed at which thedispersion of the intensity S (i, Ti) is minimized is set to the soundspeed, whereby the generated image can be generated based on a soundspeed closet to the actual sound speed. Also in the present embodiment,an increase in the background noise can be suppressed by limiting thearea where the dispersion of the intensity S (i, Ti) is minimum. Furtheraccording to the present invention, the sound speed at which thedispersion of the intensity S (i, Ti) is minimized is regarded as thesound speed, so even if noise is randomly included in the detectionsignal, the noise can be equalized (in other words, the influence ofnoise can be minimized). As a result, even if noise is included in thedetection signal, the actual sound speed can be accurately estimated.

Therefore according to the present invention, an image representinginformation inside a subject can be generated with suppressing a drop inimage quality due to noise and a mismatch of the sound speed used forgenerating the image and the actual sound speed.

EMBODIMENTS

Now an image generating apparatus and an image generating methodaccording to the present embodiment will be described. FIG. 2 shows anexample of a configuration of an image generating apparatus according tothe present embodiment. Here a case of an image generating apparatus(photoacoustic tomography apparatus) using photoacoustic tomographytechnology, which detects an acoustic wave propagating from the insideof a subject and visualizes biological information, will be described asan example. The present invention can also be applied to an imagegenerating apparatus using ultrasonic diagnostic technology (ultrasonicdiagnostic apparatus), which generates an image of informationrepresenting the inside of a subject by transmitting an acoustic wave(ultrasonic wave) from an acoustic wave probe and detecting a reflectedacoustic wave (ultrasonic wave) which is reflected inside the subject.

The image generating apparatus according to the present embodiment iscomprised of a light source 11, an optical apparatus 13, an acousticwave probe 17, a signal collector 18, an image generation processingunit 19, a signal processor 20 and a display apparatus 21. The light 12emitted from the light source 11 (intensity modulated light) isirradiated onto a subject 15, such as a living body via the opticalapparatus 13. When a part of the energy of the light propagating insidethe subject 15 is absorbed by a light absorber (sound source) 14, suchas a blood vessel, the acoustic wave (typically an ultrasonic wave) 16is generated by thermal expansion of the light absorber 14. Thegenerated acoustic wave 16 is detected by the acoustic wave probe 17,and the image of the biological information of the subject is generatedby subsequent processing.

The light source 11 emits light having a specific wavelength which isabsorbed by a specific component out of the components constituting theliving body. The light source 11 may be integrated with the imagegenerating apparatus according to the present embodiment, or may be aseparate unit. The light source 11 is constituted by one or more pulsedlight sources which can generated pulsed light in a several nano toseveral hundred nano second order. For the light source, a laser ispreferable since a laser exhibits high output, but a light emittingdiode or the like may be used instead of a laser. For the laser, variouslasers can be used, including a solid-state laser, gas laser, dye laserand semiconductor laser. Irradiation timing, wave form of light,intensity and other factors are controlled by a control unit, which isnot illustrated.

The light 12 emitted from the light source 11 is guided to the subjectvia the optical apparatus 13 (it may also be guided by an opticalwaveguide or the like). The optical apparatus 13 is, for example, amirror which reflects light, and a lens which collects or spreads light.For this optical apparatus, any apparatus can be used only if the light12 emitted from the light source can be irradiated to be a desired shapeon the subject 15. Generally it is preferable to spread the light 12 tobe a certain area, rather than collecting the light 12 by a lens, interms of safety of a living body and having a wide diagnosis area. It ispreferable that the area on which the light is irradiated on the subject(irradiation area) is movable (changeable). In other words, it ispreferable that the image generating apparatus of the present embodimentis constructed such that the light generated from the light source canmove on the subject. Then the generated image can be generated over awider range. It is preferable that the irradiated area (light irradiatedon the subject) can move synchronizing with the acoustic wave probe 17.Methods for moving the irradiation area are: using a movable mirror, andmechanically moving the light source itself among others are available.

The image generating apparatus according to the present embodiment isused for diagnosis of malignant tumors and vascular diseases of humansand animals, for example. Therefore as the subject 15, a diagnosistarget area, such as a breast, finger and limb of humans and animals canbe assumed. In the photoacoustic tomography apparatus, the lightabsorber (or sound source) 14 is a portion of the subject where anabsorption coefficient is high. For example, in the case when the humanbody is a measurement target, oxygenated hemoglobin, reduced hemoglobin,blood vessels containing a high amount of these hemoglobins, and amalignant tumor which includes many new blood vessels, for example,could be the light absorbers. In the case of an ultrasonic diagnosticapparatus, the sound source 14 is a tissue interface of which acousticimpedance is different from the surrounding area.

The acoustic wave probe 17 has a plurality of detection elements. Thedetection element is constituted by a transducer utilizing piezoelectricphenomena, a transducer utilizing the resonance of light, or atransducer utilizing the change of capacity. Configuration is notlimited to this, but can be any configuration that allows acoustic wavesto be detected. In the acoustic wave probe 17, a plurality of detectionelements are disposed typically in a one-dimensional or two-dimensionalarrangement. By using the plurality of detection elements disposed likethis, an acoustic wave can be detected in a plurality of locations allat once. Therefore the detection time can be decreased and such aninfluence as vibration of the subject can be minimized. If the acousticwave is detected in a plurality of locations by moving the position ofone detection element, signals (detection signals) similar to those inthe case of using a plurality of detection elements can be obtained.

The signal collector 18 amplifies an electric signal (analog signal)obtained by the acoustic wave probe 17 (each detection element), andconverts it into a digital signal (detection signal). The signalcollector 18 is constituted typically by an amplifier, A/D converter andFPGA (Field Programmable Gate Array) chip among other components. It ispreferable that a plurality of detection signals can be processedsimultaneously. Then the time up to generating an image can bedecreased.

The image generation processing unit 19 generates an image representinginformation inside the subject (generated image) using a plurality ofdetection signals (a plurality of digital signals which are output fromthe signal collector 18) obtained from a plurality of detection elements(the image generating unit). In each case of a general time domainmethod, each pixel of the generated image is calculated using: adetection signal of which phase is adjusted based on the distance fromeach detection element to a position corresponding to this pixel; and asound speed which is determined by the later mentioned signal processor20.

The signal processor 20 connected to the image generation processingunit 19 determines the sound speed of the acoustic wave inside thesubject (the determination unit). According to the present embodiment,the sound speed is determined using a plurality of digital signals whichare output from the signal collector 18 and the image obtained by theimage generation processing unit 19.

The display apparatus 21 is an apparatus for displaying a generatedimage generated by the image generation processing unit 19. For thedisplay apparatus 21, a liquid crystal display, a plasma display, anorganic EL display and a display having electron-emitting devices, forexample, can be used.

Now processing by the image generation processing unit 19 and the signalprocessor 20 will be described with reference to the flow chart in FIG.3.

First the image generation processing unit 19 generates a tentativegenerated image (volume data) based on the estimated sound speed(predetermined sound speed) of the subject (step 301). For a method togenerate a tentative generated image, methods conventionally used for aphotoacoustic tomography apparatus and an ultrasonic diagnosticapparatus can be used. For example, a back projection method in a timedomain or a Fourier domain can be used.

In step 301, it is preferable to use an image generating apparatus whichgenerates a generated image by detecting an acoustic wave, such as anultrasonic diagnostic apparatus and a photoacoustic tomographyapparatus. However the present invention is not limited to this, andvolume data (a generated image) on biological information may begenerated using an image generating apparatus based on a differentprinciple, such as an X-ray CT and MRI.

Then the signal processor 20 selects a pixel (target pixel) thatstrongly reflects the biological information out of the volume data(generated image) obtained in step 301, and determines an area whichincludes the position corresponding to the target pixel as the targetarea (step 302). In the case of a photoacoustic tomography apparatus,the target pixel is a pixel constituting the absorber, and in the caseof an ultrasonic generated image, the target pixel is a pixel in an areawhere reflection of the ultrasonic wave is high.

The target pixel may be manually selected by the user checking thegenerated image, or may be automatically selected. In the case ofautomatically selecting the target pixel, a pixel of which luminance orcontrast is highest in the image, for example, is selected as the targetpixel.

The size of the target area is determined based on the later mentionedselection range of the tentative sound speed. For example, a case ofobtaining a generated image using a 1500 m/sec. sound speed (estimatedsound speed c₀: predetermined sound speed), then switching the soundspeed to another sound speed (tentative sound speed) in the 1400 to 1600m/sec. range and regenerating the image, is considered. Here the pixelpitch d is 0.25 mm, and thickness r of the subject (distance from theposition in the subject corresponding to the target pixel to theacoustic wave probe) is 40 mm. In this case, it is possible that theposition corresponding to the target pixel shifts about 10 pixels at themaximum in the x, y and z directions. In such a case, an areacorresponding to the range including 10 pixels from the target pixel inthe x, y and z directions (e.g. range including 21*21*21 pixels aroundthe target pixel) is selected as the target area.

In concrete terms, a position corresponding to the target pixel in thecase of using estimated sound speed c₀ (predetermined sound speed)changes for about M pixel in width from this position if the sound speedto be used is changed. The value M is obtained by the followingExpression (102). In Expression (102), c_(min) is a minimum value of thetentative sound speed, and c_(max) is a maximum value of the tentativesound speed.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\mspace{616mu}} & \; \\{M = {\frac{c_{0}}{d}\left( {\frac{r}{c_{m\; i\; n}} - \frac{r}{c_{{ma}\; x}}} \right)}} & (102)\end{matrix}$Therefore the area corresponding to the range of width M obtained byExpression (102) around the target pixel can be set as the target area.In other words, if a three-dimensional image is generated, an areacorresponding to the range which includes M*M*M voxels around the targetpixel is selected as the target area. If a two-dimensional image isgenerated, an area corresponding to the range which includes M*M pixelsaround the target pixel is selected as the target area. Thereby an areawhich includes a sound source, such as an absorber, can be the targetarea.

The thickness r of the subject in Expression (102) is specifically adistance from a position corresponding to the target pixel to adetection element which is most distant from this position. Thethickness of the subject however is not limited to this, but may be adistance from the position corresponding to the target pixel to adetection element which is closest to this position. Or the thickness ofthe subject may be an average of the distance between the positioncorresponding to the target pixel and each detection element.

Then the signal processor 20 adjusts the phase of each detection signalbased on the distance from each detection element to the target area(e.g. a position in the target area corresponding to a pixel of thegenerated image) and a tentative sound speed. Then the signal processor20 calculates the dispersion of the intensities of a plurality ofdetection signals of which phases are adjusted (step 303). Theprocessing in step 303 is executed for a plurality of tentative soundspeeds.

Then the signal processor 20 determines a sound speed of whichdispersion of intensity is minimized, out of the plurality of tentativesound speeds, as the sound speed (step 304). The image generationprocessing unit 19 generates the generated image based on the determinedsound speed.

By the above processing, image representing information inside thesubject can be generated with suppressing a drop in image quality due tonoise and mismatch of the sound speed used for generating the image andactual sound speed.

If the area of the detection element is large, the detection signal isstrongly influenced by the directivity. Directivity will be describedwith reference to FIG. 4A and FIG. 4B. Directivity is a characteristicwhere the detection sensitivity of the detection element changesdepending on the relative positional relationship between the detectionelement and a position inside the subject (position corresponding to thepixel: processing target position). For example, if the detectionelement is facing the processing target position as shown in FIG. 4A,the sensitivity to detect the acoustic wave from this position is at themaximum (intensity of the detection signal is at the maximum). As thedeviation between the direction of the detection element and theprocessing target position becomes greater, the detection sensitivitydecreases (intensity of the detection signal decreases). This influenceof directivity becomes conspicuous when the width of the element islarge, or when the frequency of the acoustic wave to be detected ishigh. Directivity R (detection sensitivity) is given by the Expression(103).

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\mspace{616mu}} & \; \\{R = {\frac{\sin\left( {{k \cdot d}\;\sin\;\theta} \right)}{{k \cdot d}\;\sin\;\theta}}} & (103)\end{matrix}$

-   d: width of detection element-   θ: angle formed by the line connecting the processing target    position and the detection element and the direction of the    detection element-   k: 2π/λ-   λ: wavelength of acoustic wave    The shape of the detection element is assumed to be a square.

FIG. 4B shows an example of the directivity of the detection element ofwhich the element width is 2 mm. Here a magnitude of each frequencycomponent is calculated using the frequency characteristic obtained fromthe impulse response of the detection element, and a directivity(detection sensitivity standardized with maximum value 1) is calculatedusing Expression (103). As FIG. 4B shows, the sensitivity (intensity ofdetection signal) suddenly drops if the angle formed by the lineconnecting the processing target position and detection element and thedirection of the detection element increases. Such directivity increasesdispersion among detection signals.

Therefore it is preferable that the image generating apparatus of thepresent embodiment further has a function to correct the drop inintensity of the detection signal due to the directivity of thedetection element, according to the relative positional relationship ofthe position corresponding to the pixel and detection element. And it ispreferable that the signal processor 20 determines the sound speed usingthe detection signal of which intensity is corrected, and the imagegeneration processing unit 19 generates the image using the detectionsignal, of which intensity is corrected. Thereby the influence ofdirectivity of the detection element can be suppressed. For example,dispersion of the detection signals due to directivity can be decreased(dispersion due to sound speed becomes dominant), so the sound speed canbe determined more accurately. Also the dispersion of intensity of thedetection signal due to directivity is decreased, so an image of whichimage quality does not drop can be generated.

For correction, the angle formed by the line connecting the processingtarget position and the detection element and the direction of thedetection element is calculated based on the relative positionalrelationship of the processing target position and the detectionelement, for example, and intensity of the detection signal, which isoutput from the detection element, is increased based on the value ofthe directivity according to the angle (e.g. value of the Y axis in FIG.4B). In concrete terms, the detection signal is multiplied by theinverse number of the value of directivity. Then dispersion of intensityof the detection signal due to directivity can be suppressed.

In many cases, the processing of the image generation processing unit 19and the signal processor 20 are implemented by the processing unit of acomputer (e.g. workstation) executing software (programs). Therefore theimage generation processing unit 19 and the signal processor 20 may notbe distinguished. These functional elements may be constructed by adedicated chip respectively.

The processing of the image generation processing unit 19 and the signalprocessor 20 may be implemented by hardware instead of software.

According to the present embodiment, the target area is determined usingthe position of the target pixel and Expression (102), but the presentinvention is not limited to this. If a general location of the targetarea can be estimated, such as the case when a predetermined subject ismeasured, information on the target area may be stored in the apparatusin advance. Information on the predetermined target area may be storedin advance for each type (measuring area) of subject. In such a case,the tentative generated image in step 301 of the present embodiment neednot be generated. The target area can be any size (even a one pixelsize). For example, the target area can be set so that the positioncorresponding to the target pixel selected in step 302 is included.Since the sound source is assumed to have a certain size, the targetarea set like this can receive sound from the sound source even if theposition corresponding to the target pixel is shifted to outside thetarget area by a change in sound speed.

EXAMPLE 1

The present invention is simulated, and the effect thereof is verified.This example will be described with reference to FIG. 5A, FIG. 5B andFIG. 6.

As a subject, a cubic phantom 43 of which X direction: 4 cm, Ydirection: 4 cm and Z direction: 4 cm, is used, as shown in FIG. 5A. Atthe center of the cubic phantom 43, a cylindrical sound source 41, ofwhich diameter is 0.05 cm and height is 2 cm, is disposed so that theaxis direction thereof is parallel with the X direction. Here the soundsource 41 is a light absorber in the case of photoacoustic tomography,and an ultrasonic reflector in the case of an ultrasonic diagnosticapparatus. It is assumed that the sound speed of the acoustic waveinside the subject is 1500 m/sec., and the initially generated soundpressure, which is generated in the sound source 41, is 3000 Pa(pascal).

The acoustic wave probe 42 is a two-dimensional array type and isconstituted by 20*20 square detection elements. The length of one sideof each element is 2 mm, and the pitch distance is 2 mm. It is assumedthat the acoustic wave is detected with a sampling frequency of 20 MHzat 1280 measurement points.

Under the above conditions, the detection signal of the acoustic wavedetected by each element of the acoustic wave probe 42 is generated byphysical simulation. FIG. 5B shows an example of the detection signalobtained by the detection element at the center of the probe. The strongsignal in FIG. 5B indicates a signal from the sound source 41. It isassumed that the detection signal includes a white noise, in which thetripled standard deviation is 20 Pa.

Using a detection signal obtained in this way and a sound speed(predetermined sound speed) of 1580 m/sec., a generated image isgenerated. For the generation method, a universal back projectionaccording to a time domain method, which is a publicly known technology,is used. The imaging range is X direction: 3.8 cm, Y direction: 3.8 cmand Z direction: 4 cm, and the pixel pitch (voxel pitch) is 0.025 cm. Atotal number of voxels is acquired by dividing the imaging range by thevoxel pitch, that is 152*152*160.

FIG. 6 shows the generated images. All the images shown in FIG. 6 areMIP (Maximum Intensity Projection) images in which maximum intensity ofvoxel values are projected in the direction of the acoustic wave probe42, and the white area indicates high sound pressure. The change ofsound pressure of the image in the X direction at Y=2.0 cm is shownabove each image.

In FIG. 6, the image A shows an MIP image of the sound source inside thesubject. According to this example, it is assumed that the resolution ishigher as the image is closer to image A.

Image B is a generated image obtained when a 1580 m/sec. sound speed isused as the sound speed. As this image shows, if a sound speed differentfrom the actual sound speed of 1500 m/sec. is used, the image blurs andresolution drops.

Then in this image (volume data), a voxel of which luminance is maximumis searched. As a result, the voxel at the center (76*76*80^(th) voxelin all the voxels of 152*152*160) is detected. The area corresponding to21*21*21 voxels centering around this voxel is set as the target area.

Then in order to compare with prior art, the luminance value iscalculated for each position (voxel) in the target area, while changingthe sound speed to be used from 1400 m/sec. to 1600 m/sec. at a 2 m/sec.interval. In other words, 21*21*21*101 number of luminance values arecalculated. As a result, the sound speed at which the luminance value ismaximized is 1456 m/sec. Then using this sound speed 1456 m/sec., animage is generated again. Image C is an image obtained at this time. Theimage is somewhat blurred, but resolution is slightly improved comparedwith image B. The maximum luminance value also increases.

In order to substantiate the effect of the present invention, thedispersion of intensity (index CF) of the detection signal is calculatedfor each position (voxel) in the target area while changing the soundspeed to be used from 1400 m/sec. to 1600 m/sec. at a 2 m/sec. interval.In other words, 21*21*21*101 number of index CFs are calculated. As aresult, the sound speed at which the index CF is maximized is 1492m/sec. Then using this sound speed of 1492 m/sec., the image isgenerated again. Image C′ is an image obtained at this time. Comparedwith image C, which is the prior art, image blur is less and resolutionimproves.

Table 1 shows this result.

TABLE 1 Image quality Velocity (amount of blur) Image A (actual image)1500 m/sec. None Image B (estimate) 1580 m/sec. High Image C (prior art)1456 m/sec. Mid- Image C′ (this example) 1492 m/sec. Low

In this way, according to the present invention, a sound speed close tothe actual sound speed (1492 m/sec.) can be estimated. Particularly in adetection signal containing noise, a sound speed closer to the actualsound speed can be estimated than the sound speed obtained by the priorart (1456 m/sec.). As a result, an image without a drop in resolutioncan be obtained.

The sound speed is estimated as 1492 m/sec. while the actual sound speedis 1500 m/sec., because of the influence of noise and directivity of thedetection element.

In this example, dispersion of the intensity of the detection signals iscalculated for each position in the target area, but the dispersion maybe calculated for a part of the positions in the target area. Forexample, the dispersion may be calculated only for the center positionof the target area.

EXAMPLE 2

An example of applying the present invention to a photoacoustictomography apparatus will now be described with reference to FIG. 2. Inthis example, a Q switch YAG laser, which generates a pulsed light witha 1064 nm wavelength for about 10 nano seconds, is used as a lightsource 11. The energy of the pulsed laser beam (light 12) is 0.6 J, andthis pulsed light spreads to about a 2 cm radius using an opticalapparatus 13 such as a mirror and a beam expander. For a subject 15, aphantom simulating a living body is used. The phantom used is 1%intralipid solidified with agar. The size of the phantom is width: 12cm, height: 8 cm and depth: 4 cm. In this phantom, a black rubber wirewith a 0.03 cm diameter is embedded as a light absorber 14 in the centerarea. Light 12, which is spread to a 2 cm radius, is irradiated ontothis phantom. For the acoustic wave probe 17, an ultrasonic transducermade from PZT (lead zirconate titanate) is used. This transducer is atwo-dimensional array type, where a number of elements is 18*18, theshape of each element is square, and the element pitch is 2 mm. Thewidth of each element is about 2 mm. This acoustic wave probe can movein the X and Y directions synchronizing with the light irradiation area,so as to image a large area.

If a pulsed light is irradiated onto one surface of the phantom, thelight diffused inside the phantom is absorbed by the rubber wire, and aphotoacoustic wave is generated. This photoacoustic wave is detected byeach ultrasonic transducer having 324 channels, and the digital data(detection signal) of the photoacoustic wave is obtained for eachchannel, using a signal collector 18, which is comprised of anamplifier, A/D converter and FPGA. In order to improve the S/N ratio ofthe signal, the laser is irradiated 36 times, and all obtained detectionsignals are averaged. Then the obtained digital data is transferred to aworkstation (WS), which is an image generation processor 19 and a signalprocessor 20, and the WS stores this data. After performing noisereduction processing on this stored data by discrete wavelettransformation, the image is generated using a 1540 m/sec. sound speed,which is an average sound speed of an acoustic wave inside living body.Here volume data is generated using a universal back projection method,which is a time domain method. The voxel pitch used in this case is 0.05cm. The imaging range is set to 11.8 cm*11.8 cm*4.0 cm. FIG. 7A shows anexample of the obtained image.

Then the dispersion of intensity (index CF) of the detection signal iscalculated for each position (voxel) in the target area, while changingthe sound speed to be used from 1400 m/sec. to 1600 m/sec. at a 2 m/sec.interval. According to this example, in the obtained volume data, anarea corresponding to the voxel group (21*21*21 voxels), which includesthe sound wave (initial sound pressure) generated in the wire, is set asthe target area. Also according to this example, the detection signal iscorrected considering the directivity calculated based on the impulseresponse of the detection element, and the index CF is calculated usingthe corrected detection signal. The maximum index CF improved from 0.056to 0.078 by considering the directivity. In this example, the soundspeed at which the index CF becomes maximum is 1454 m/sec. Then usingthis 1454 m/sec. sound speed and the detection signal stored in a PC, animage is generated again. The obtained image is displayed on a liquidcrystal display, which is a display apparatus 21. FIG. 7B shows anexample of the obtained image.

As the above result shows, the width of the acoustic wave (initial soundpressure) generated from the rubber wire is obviously narrower in theimage generated using a 1454 m/sec. sound speed (FIG. 7B) than in theimage generated using a 1540 m/sec. sound speed (FIG. 7A). It is clearthat there is hardly any image blur, in other words, resolutionimproved.

In this way, according to this example, the sound speed of the acousticwave inside the subject can be accurately estimated, and thereforeresolution of the generated image can be improved.

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., non-transitory computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

The invention claimed is:
 1. An apparatus for generating a photoacousticimage based on an acoustic wave caused by irradiation of light on anobject, comprising: a determination unit configured to determine apropagation velocity of the acoustic wave; and an image generation unitconfigured to generate the photoacoustic image based on the propagationvelocity and a signal obtained by detecting the acoustic wave; whereinthe determination unit is configured to set, based on an ultrasonicimage obtained by transmitting and receiving an ultrasonic wave, atarget position, and determine, based on the signal corresponding to thetarget position, the propagation velocity.
 2. The apparatus according toclaim 1, wherein the determination unit is configured to set the targetposition based on an instruction from a user for the ultrasonic image.3. The apparatus according to claim 2, wherein the determination unit isconfigured to set a target area including the target position selectedby the user in the ultrasonic image, and determine the propagationvelocity based on a signal corresponding to the target area from among aplurality of the signals.
 4. The apparatus according to claim 3, whereinthe determination unit is configured to set, as the target area, an areain a range of a predetermined number of pixels or voxels from the targetposition.
 5. The apparatus according to claim 3, wherein the imagegeneration unit is configured to generate the photoacoustic image on thetarget area.
 6. The apparatus according to claim 1, wherein thedetermination unit is configured to set the target position by analyzingthe ultrasonic image.
 7. The apparatus according to claim 6, wherein thedetermination unit is configured to set the target position based on aluminance value of the ultrasonic image.
 8. The apparatus according toclaim 7, wherein the determination unit is configured to set, as thetarget position, a position having the highest luminance value in theultrasonic image.
 9. The apparatus according to claim 6, wherein thedetermination unit is configured to set the target position based oncontrast in the ultrasonic image.
 10. The apparatus according to claim9, the determination unit is configured to set, as the target position,a position having the highest contrast in the ultrasonic image.
 11. Theapparatus according to claim 6, wherein the determination unit isconfigured to set, as the target position, a position including a bloodvessel in the ultrasonic image.
 12. The apparatus according to claim 6,wherein the determination unit is configured to set a target areaincluding the target position, and determine the propagation velocitybased on a signal corresponding to the target area from among aplurality of the signals.
 13. The apparatus according to claim 1,wherein the determination unit is configured to set a tentativepropagation velocity, and determine a signal corresponding to the targetposition from among a plurality of the signals based on a distance froma detection position of the acoustic wave to the target position and thetentative propagation velocity.
 14. The apparatus according to claim 13,wherein the determination unit is configured to execute a process ofdetermining the signal corresponding to the target position based on thedistance from the detection position of the acoustic wave to the targetposition and the tentative propagation velocity, for a plurality of thetentative propagation velocities, and determine the propagation velocitybased on a plurality of the signals corresponding to the plurality ofthe tentative propagation velocities.
 15. The apparatus according toclaim 1, further comprising: a light irradiation unit configured toirradiate the object with the light; and an acoustic wave detection unitconfigured to output the signal by detecting the acoustic wave.
 16. Theapparatus according to claim 15, wherein the light irradiation unitincludes a semiconductor laser or a light emitting diode.
 17. Theapparatus according to claim 1, wherein the image generation unit isconfigured to generate the photoacoustic image as three-dimensionalvolume data.
 18. The apparatus according to claim 1, wherein thedetermination unit is configured to set the target position based on theultrasonic image which is two-dimensional pixel data.
 19. An imagegenerating method for generating a photoacoustic image based on anacoustic wave caused by irradiation of light on an object, comprising: asetting step of setting, based on an ultrasonic image obtained bytransmitting and receiving an ultrasonic wave, a target position; adetermination step of determining, based on a signal corresponding tothe target position from among a plurality of signals which have beenobtained by receiving a plurality of the acoustic wave, a propagationvelocity of the acoustic wave; and a generating step of generating thephotoacoustic image based on the signal and the propagation velocity.20. The image generating method according to claim 19, wherein, in thedetermination step, the target position is set based on an instructionfrom an user for the ultrasonic image.
 21. The image generating methodaccording to claim 20, wherein, in the determination step, an areaincluding the target position selected by an user in the ultrasonicimage is set as a target area, and the propagation velocity isdetermined based on a signal corresponding to the target area from amongthe plurality of the signals.
 22. The image generating method accordingto claim 21, wherein, in the determination step, an area in a range of apredetermined number of pixels or voxels from the target position is setas the target area.
 23. The image generating method according to claim21, wherein, in the generating step, the photoacoustic image on thetarget area is generated.
 24. The image generating method according toclaim 19, wherein, in the setting step, the target position is set byanalyzing the ultrasonic image.
 25. The image generating methodaccording to claim 24, wherein, in the setting step, the target positionis set based on a luminance value of the ultrasonic image.
 26. The imagegenerating method according to claim 25, wherein, in the setting step, aposition having the highest luminance value in the ultrasonic image isset as the target position.
 27. The image generating method according toclaim 24, wherein, in the setting step, the target position is set basedon contrast in the ultrasonic image.
 28. The image generating methodaccording to claim 27, wherein, in the setting step, a position havingthe highest contrast in the ultrasonic image is set as the targetposition.
 29. The image generating method according to claim 24,wherein, in the setting step, a position including a blood vessel in theultrasonic image is set as the target position.
 30. The image generatingmethod according to claim 24, wherein, in the determination step, atarget area including the target position is set, and the propagationvelocity is determined based on a signal corresponding to the targetarea from among a plurality of the signals.
 31. The image generatingmethod according to claim 19, wherein, in the determination step, atentative propagation velocity of the acoustic wave is set, and a signalcorresponding to the target position is determined from among aplurality of the signals based on a distance from a detection positionof the acoustic wave to the target position and the tentative velocity.32. The image generating method according to claim 31, wherein, in thedetermination step, a process of determining the signal corresponding tothe target position based on the distance from the detection position ofthe acoustic wave to the target position and the tentative propagationvelocity is executed for a plurality of the tentative propagationvelocities, and the propagation velocity is determined based on aplurality of the signals corresponding to the plurality of the tentativepropagation velocities.
 33. The image generating method according toclaim 19, wherein, in the generating step, the photoacoustic image isgenerated as three-dimensional volume data.
 34. The image generatingmethod according to claim 19, wherein, in the determination step, thetarget position is set based on the ultrasonic image which istwo-dimensional pixel data.
 35. A non-transitory computer readablemedium recording a program for causing a computer to perform each stepsof the image generating method according to claim 19.